PracticalUseofSoilMoistureSensorsforIrrigationSchedulingR.TroyPeters,P.E.,Ph.D.

Goodirrigationwatermanagementwillincreaseyields,improvecropquality,conservewater,saveenergy,decreasefertilizerrequirements,andreducenonpointsourcepollution.Usingsoilmoisturemeasurementsisoneofthebestandsimplestwaystogetfeedbacktohelpmakeimprovedwatermanagementdecisions.However,theinstallation,calibration,andinterpretationofthedatafromtheseinstrumentsisoftenoverwhelmingformostbusygrowers.Heresanattempttoprovidepracticalrecommendationsforusingthesesensorstoimproveyouroperation.ThemajortypesofsoilmoisturesensorsarelistedinTable1andgroupedaccordingtothetechnologyusedtomeasuresoilmoisture.Researchcontinuestoshowthatthesesensorsarenotalwaysaccurate.However,theyallgivetrendlinesthatcanbeusableforirrigationscheduling.Althoughthetechnologiesusedbyeachsensortypearequitedifferent,thesesensorscanberoughlycategorizedintotwogroups:thosethatgivethesoilwatercontent,andthosethatgivethesoilwatertension.Table1.Majortypesofsoilmoisturesensorsandtheirrelativeadvantagesanddisadvantages.

SensorType Advantages Disadvantages

Soil W

ater C

onten

t

NeutronProbe(CampbellPacificNuclear;CPN)

Accurate.Repeatable.Samplesarelativelylargearea.Onesensorforallsites&depths.

Governmentrequiredpaperworkandregulations.Can'tleaveinfield.Relativelyexpensive(about$4,500).

TimeDomainTransmissivity(Acclima,GroPoint)

Lessexpensive($110/sensor).Easytologdata.

Samplessmallarea.

CapacitanceSensors(Enviroscan,EchoProbes,Acclima,Vernier,etc..)

Easytosetuptologand/ortransmitdata.

Highlyaffectedbysoilconditionsimmediatelynexttothesensor.Highvariability.Moreexpensive($300$1,200/system).

Soil W

ater T

ensio

n Tensiometers Lessexpensive($80/sensor)

Maintenanceissues.

GranularMatrixSensors(Watermark)

Inexpensive($40/sensor) Highlyvariableoutput.Lessaccurate.Sensitivetotemperatureandsoilsalinity.

SoilWaterContentbasedsoilmoisturesensors:(Capacitance,NeutronProbe,gravimetric)Soilwatercontentmeasurementsaremuchmoremeaningfulforirrigationschedulingwhentheyarecomparedtothemaximumamountofwaterthatthesoilcanholdlongterm,orfieldcapacity.Thesimplestwaytodetermineyoursoilsfieldcapacityistousethesensortotakeasoilwatercontent

measurementatatimewhenyouareconfidentthatthesoilisfullofwater,yetfreewaterhashadtimetodrainthrough.Goodtimestotakethesemeasurementsareinthespringassoonassoilthaws(assumingadequatesoilmoisturerechargeoverthewinter),or12to24hoursafteraheavyirrigation.Rememberthatthesoilcontentmeasurementmustbemultipliedbythedepthofsoilintherootzonethatitrepresentstogivethetotalwatercontentinthatsoildepth.Italsohelpsgreatlytohaveanestimateofthesoilwatercontentatwhichtheplantsbegintoexperiencewaterstress.Thiscanbeestimatedfromthepreviouslymeasuredfieldcapacityandthesoilsavailablewatercapacity(AWC).ArangeofAWCsbysoiltextureisgiveninTable2(seehttp://websoilsurvey.nrcs.usda.govforamoreaccurateestimate).ThisAWCnumberisthenmultipliedbya3.5ftrootdepthformostfruittreesandvines(secondcolumnofnumbersinTable2)togettheinchesofwaterthatisheldbetweenfieldcapacityandwiltingpoint.Itmaybenecessarytouseadifferentrootdepthbeside3.5ftifthesoilisshallow.Tomanagefruittreesandvinesfornostressitisagoodideatolimitthesoilwaterdepletiontoonly50%(half)oftheAWCintherootzone.ThisisgivenasthelastcolumnofnumbersinTable2.Now,forexample,ifthetotalwaterheldintherootzoneatfieldcapacityismeasuredinthespringafterawetwintertobe7.0inchesofwaterinasandyloamsoil(maximumof2.5inchesdepletionfornostressfromTable2),wesetthemaximumdepletionpointto4.5inches(7.02.5in).Plantandsoilobservationsshouldalsobeusedasfeedbacktorefinetheseestimatesovertime.Forinstance,ifintheaboveexampleIsawthefirstsignsofvinewaterstressatameasured5inchesofwaterintherootzoneinsteadof4.5inches,Iwouldbesuretoirrigatebeforethesoilgotto5inchesofwaterinthefuture.Table2.Rangesofavailablewaterbysoiltexture.

SoilTexture Avail.WaterCapacity(AWC)in/ft

TotalWaterin3.5ftRootZone(in)

Max.Depletionat50%ofAWC(in)

CoarseSand 0.20.8 0.72.8 0.41.4FineSand 0.71.0 2.53.5 1.21.8LoamySand 0.81.3 2.84.6 1.42.3SandyLoam 1.11.6 3.95.6 1.92.8

FineSandyLoam 1.22.0 4.27.0 2.13.5SiltLoam 1.82.5 6.38.8 3.24.4

SiltyClayLoam 1.61.9 5.66.7 2.83.3SiltyClay 1.52.0 5.37.0 2.63.5Clay 1.31.8 4.66.3 2.33.2

PeatMucks 1.92.9 6.710.2 3.35.1Usingthismethodtheabsoluteaccuracyofthesensorislessimportantbecausewearejustcomparingittoitself.Alsotheperiodicmeasurementsthroughouttheseasonnowtakeonmeaningaswecandeterminethesoilwaterdeficit(fieldcapacitythecurrentreading)andtheamountofwaterthatcanbedepletedbeforewaterstressoccurs.Goodirrigationmanagerswillmaintainthewatercontentwellbetweenfieldcapacityandthisstresspoint.

Tensionbasedsoilmoisturesensors:(GMS,Tensiometers)Whenusingtensionbasedsoilmoisturesensors,thesoilsfieldcapacity,wiltingpoint,andthemaximumdepletionpointaremostlyirrelevant.Asoilthatisfullofwaterwillhaveameasuredsoilwatertensionnearzero.Fruittreesandvinesshouldbeirrigatedbeforetheyreach4050centibars.Forregulateddeficitirrigation,thiscouldbeincreasedto80centibars.Again,sincethesemeasurementscanbeinaccurateandsoilspecific,refineyourlimitsusingcropobservationsovertime.Forexample,notethemeasuredsoilwatertensionattheearliestindicationsofwaterstress(thiswillappearfirstinsandy,orshallowsoilareas),andbesuretoirrigatebeforeyoureachthatpointinthefuture.Alsotakesomereadingsrightafteranirrigation.Ifthebottomsensorgoestozero,thenitspossibleyouputtoomuchwateron.Ifitshowsnomovementatall,applymorewaternexttimetopushwaterabitdeeper.AdditionalRecommendations:

Avoidpreferentialflowofwatertothesensorduetoinstallationprocess. Flagthesensorwellsothatitcanbeeasilyfound. Graphicalrepresentationofthedatagreatlyhelpswithdatainterpretation. UsesoilwatermeasurementswithirrigationschedulingtoolssuchasKanschedanddailywater

usedatafromAgWeatherNetorAgriMetformuchbetterwatermanagement(allthreeofthesewillbethetopsearchreturnonGoogle).

Keeprecords.Correlatereadingswithobservations. Stayawayfromboththefieldcapacity,andwaterstresspointsifpossible. Realizethatsoilandsensorshavealotofvariability. Bepatientandstickwithit.Itmaytakeayearortwobeforeyouaregoodatinterpretingyour

sensorreadings.